1. Field of the Invention
The present invention relates generally to power headroom reporting and, more particularly, to methods for resource allocation and power control based on power headroom reporting.
2. Description of the Related Art
In recent years, in order to achieve high-speed data transmission over radio channels of mobile communication systems, significant research efforts have been made to develop technologies related to Orthogonal Frequency Division Multiplexing (OFDM) and Single Carrier-Frequency Division Multiple Access (SC-FDMA). For example, the Long Term Evolution (LTE) system, which is regarded as a next-generation mobile communication system, employs OFDM for downlink and SC-FDMA for uplink. However, since OFDM has a high Peak-to-Average Power Ratio (PAPR), a large back-off is required for the input to the power amplifier to avoid nonlinear signal distortion, which lowers the maximum transmit power. This results in low power efficiency. The back-off sets the maximum transmit power to a level lower than the maximum power of the power amplifier, in order to ensure linearity of the transmit signal. For example, when the maximum power of the power amplifier is 23 dBm and the back-off is 3 dBm, the maximum transmit power becomes 20 dBm. OFDMA does not have any significant drawbacks as a downlink multiplexing technology, because the transmitter is located in a base station that has no power limitations. However, OFDMA has significant drawbacks, as an uplink multiplexing technology, because the transmitter is located in user equipment (such as a mobile terminal), which has severe power limitations. These limitations may reduce the terminal transmit power and service coverage. Consequently, SC-FDMA has been employed as uplink multiplexing technology for LTE, which is proposed by 3GPP (3rd Generation Partnership Project) as a fourth generation mobile communication system.
High-speed data transmission is required to provide diverse multimedia services in advanced wireless communication environments. In particular, considerable effort has been made to develop Multiple-Input and Multiple-Output (MIMO) technology for high-speed data transmission. MIMO employs multiple antennas to increase channel capacity within given frequency resource limitations. In scattering environments, use of multiple antennas may produce a channel capacity proportional to the number of antennas. Precoding is necessary in order to efficiently transmit data through MIMO. Precoding rules may be represented in a matrix form (precoding matrices), and a set of pre-defined precoding matrices is referred to as a codebook. In LTE Advanced (LTE-A), MIMO based on precoding matrices is recommended as a primary uplink technology enabling performance enhancement in both single-user and multi-user environments.
However, several problems exist when using an LTE-A system. First, the LTE-Advanced system allows simultaneous transmission over a Physical Uplink Shared Channel (PUSCH) and a Physical Uplink Control Channel (PUCCH). When a mobile terminal capable of simultaneous transmission sends a power headroom report containing only PUSCH transmit power information to a serving base station, the base station may adjust an amount of allocated PUSCH transmit power and other resources than are actually needed to the mobile terminal. In other words, cell interference may be increased due to excessive transmit power, or terminal link performance may be lowered due to insufficient transmit power.
Second, when a base station in an LTE-Advanced system schedules a MIMO transmission for a mobile terminal using the LTE scheduler, the base station may use the Sounding Reference Signal (SRS) from the mobile terminal to select precoding matrices that maximize uplink channel capacity. In MIMO transmission, one codeword may be mapped to transmission layers in different channel environments. However, a transmit power of each layer may be not adjusted using precoding matrices alone.